Abstract:

For the past six years a bacterial infection has been the cause
of large losses of adult, spawning, American shad (Alosa
sapidissima) in the Coos, Millicoma and Smith Rivers of Oregon.
There was a sizable commercial fishery for shad in these rivers
and losses of fish due to this infection represented an important
economic loss to the industry. This study was undertaken to
determine the causative agent of the disease and describe the
pathology of the disease.
Fifteen strains of bacteria were isolated from diseased shad
in the Coos, Millicoma, and Smith Rivers. Cultural and morphological
studies, physiological reactions, animal infection, serological
tests, and determination of the mole percent guanine +
cytosine showed that the causative agent belonged in the genus
Aeromonas. Since the shad disease isolates could cause "red leg"
disease in frogs and "red mouth" disease in trout, they were
named Aeromonas hydrophila.
The disease was shown to be a bacterial hemorrhagic septicemia.
Externally, the diseased shad had large hemorrhagic
areas on their sides and reddening of the head and fins. Very little
pathology could be seen internally. The causative organism was
recovered from the spleen, liver, kidney, heart, blood and external
lesions of the diseased animals.
A review of the literature revealed that there was confusion in
the nomenclature of the three motile species of bacteria included
in the genus Aeromonas. Many authors felt that the three species
(A. liquefaciens, A. hydrophila, A. punctata) were one distinct
species. Comparative tests performed on the shad disease isolates
and known cultures of Aeromonas failed to reveal major differences
between these organisms.
Attempts were made to separate the three species on the basis
of the mole percent guanine + cytosine and thermal denaturation
temperature (Tm) of the bacterial DNA. Three shad disease isolates
had Tm values of 94.3, 94.6, and 94.2°C with corresponding
mole percent guanine + cytosine values of 60.9, 61.7, and 60.7.
An isolate of Aeromonas liquefaciens obtained from the Communicable
Disease Center (CDC) in Atlanta, Georgia had a Tm of
94.1°C and a mole percent guanine + cytosine of 60.3.
Aeromonas hydrophila (CDC) had a Tm of 95.4°C and a mole percent
guanine + cytosine of 63.4 while A. punctata (CDC) had a Tm
of 93.5°C and a mole percent guanine + cytosine value of 59.0.
The mole percent guanine + cytosine results indicated that the shad
disease bacterium was closely related to A. liquefaciens. The
three known cultures of Aeromonas each had a distinct mole percent
guanine + cytosine value.